A commonly used pulp or fiber refiner comprises a rotor unit and a stator unit (or alternatively, two rotor units) that are aligned along a common axis and facing each other, for grinding lignocellulose-containing material, such as wood chips, into pulp. The refining of the pulp/fiber is performed in a bounded area between the rotor unit, or rotor, and the stator unit, or stator. FIG. 1 is a schematic illustration of a part of an embodiment of a pulp/fiber refiner 1 viewed from above. During use of the pulp/fiber refiner 1 of FIG. 1 lignocellulose-containing material 7, such as wood chips, is fed into the preheater 2. Steam 8 is input at the bottom of the preheater 2 and goes upwards through the pile of wood chips. The wood chips are discharged from the preheater 2 by a discharge screw 2a and fed into a feed screw 3a which feeds the chips via a feeding channel 3 towards the defibrator 4. The wood chips are fed by the feed screw 3a through a hole in the stator 5 to emerge in an area bounded by the stator 5 and the rotor 6. The rotor 6 facing the stator 5 is arranged on a rotatable axis that can be rotated by means of an electrical motor. The purpose of the rotor is to grind the lignocellulose-containing material between a surface of the stator and a surface of the rotor. Thus, when lignocellulose-containing material leaves the feeding channel and enters the bounded area, or refining gap/disc gap, between the rotor and the stator it flows in on the rotor and due to the rotation of the rotor the lignocellulose-containing material, such as wood chips/fiber/pulp, is directed outwards towards the periphery of the rotor and stator. Usually there are provided refining segments on the surfaces of the rotor and/or the stator. The purpose of these refining segments is to achieve a grinding action on the pulp/fiber.
The lignocellulose-containing material should be fed through the refiner as evenly as possible in order to save energy and promote an even grinding of the pulp/fiber. Usually the material feed in a refiner typically varies with time t in a more or less periodic fashion as schematically illustrated in FIG. 2A. Ideally these feed variations should be kept at a minimum to save energy and improve fiber quality. It is therefore important to achieve an even feed into the feed screw, as well as minimal disturbance from back-streaming steam from the defibrator, as will be described further below.
The defibration difficulty of each individual wood piece fed into a refiner also typically varies with time t as schematically illustrated in FIG. 2B, and these variations should also be kept at a minimum. The defibration difficulty per wood piece typically depends on e.g. wood density, wood moisture, chip size, cooking condition etc.
One problem with common refiner designs is that the chips/fiber/pulp will be directed towards the periphery of the rotor and stator in an uneven fashion. Large chunks of material will be localized in some positions of the rotor/stator arrangement while other positions will be more or less devoid of material. This will in turn lead to uneven grinding of the pulp/fiber. Thus, efforts have to be made to improve the distribution of the material.
Another problem within the art is that part of the lignocellulose-containing material initially can get stuck in the middle of the rotor. This might lead to material piling up in the middle of the rotor which can negatively affect the pulp/fiber distribution. A known measure to achieve a more even pulp/fiber distribution is to provide the rotor surface with a center plate 10, as illustrated in FIG. 3. The purpose of the center plate is to help feeding the lignocellulose-containing material 7 towards the periphery of the rotor 6 and stator 5. Such a center plate is typically provided with a set of feeding bars or “wings” or wing profiles, whose purpose is to direct the chips/fiber/pulp more evenly towards the rim of the stator/rotor arrangement. An example of a prior art center plate 10 with feeding wings 100 is schematically illustrated in FIG. 4. The wings are usually elongated protrusions provided on the surface 200 of the center plate of the rotor, where the surface 200 is facing the incoming material flow. The wings are usually curved e.g. in an arc-shaped form, but straight wings are also possible. By means of such wings pulp/fiber will be directed into the open channels defined between adjacent wings to thereby give a more even distribution of the pulp/fiber in the refining area. The center plate can have different amount of wings, and the wings may have different angles on the center plate, but the wings are always arranged in such a way that the feeding angle of the wings enable feeding of the lignocellulose-containing material towards the periphery of the center plate, depending on the direction of rotation of the rotor and center plate. The feeding angle of a feeding wing is defined by the angle between the leading edge of the wing at a given point and a radial line passing through that point. The leading edge is the edge of the feeding wing directed in a same direction as the direction of rotation, and the feeding angle has a positive value in a direction opposite to the direction of rotation. Thus, a feeding angle that enables feeding of the material towards the periphery of the center plate is >0° but <90°.
This is illustrated in FIG. 4, where a rotation of the rotor and center plate 10 in the direction of rotation 11 will cause at least part of the lignocellulose-containing material 7 to flow along the feeding wings 100 in a direction towards the periphery of the center plate 10. Prior art feeding wings commonly go all the way from the center to the periphery of the center plate.
WO 2014/142732 A1 shows a center plate for a rotor in a pulp refiner. The center plate has a surface provided with a plurality of first wings for directing pulp flowing onto the center of the center plate towards the periphery of the plate, where the surface is a flat surface or a surface with a central protuberance and where each of the first wings is an arc-shaped protrusion extending between a corresponding first point and a corresponding second point on the surface. The first point is displaced from the center point of the plate and the second point is arranged further from the center point than the first point. The first wings are given an arc-shape that yields a larger pulp feeding angle than a circular arc intersecting the center point of the center plate and ending in the same corresponding second point.
However, there is continued need in the art to further improve the pulp/fiber distribution in a pulp/fiber refiner. Therefore, there is still a need for a feeding center plate which further improves the pulp/fiber distribution in the refining area of a pulp/fiber refiner.